ZooKeeper plugin

ZooKeeper is a centralized server used for maintaining configuration information,
naming, providing distributed synchronization, and providing group services.
All of these kinds of services are used in some form or another by distributed applications.
For more information visit zookeeper.apache.org.

Discovery

Even if the Zookeeper plugin is not configured, Monitoring will discover ZooKeeper services running in your Cloud Platform project by:

searching instance names for zookeeper

checking for ports opened to 2181 via firewall rules

The services discovered are displayed on the ZooKeeper Services page in the Resources menu.
After you install the monitoring agent and configure the ZooKeeper plugin on your instances,
Monitoring populates the ZooKeeper Services page with inventory and metrics.